1. Technical Field
The present invention is directed to measuring the resistivity of an earth formation traversed by a borehole. More particularly, the pi is directed to measuring the resistivity of the formation located in the region relatively close to the wall of the borehole, i.e., the region which has been invaded by mud filtrate as a result of drilling the borehole.
2. Background Information
During drilling operations, it is common to maintain a hydrostatic pressure produced by the column of mud filling the borehole at a value which is slightly greater than the pressure of the earth formations. This gives rise to the invasion of mud filtrate into the formation along with a deposit of mud cake on the wall of the borehole. See, e.g., U.S. Pat. No. 4,594,552 to Grimaldi, et al., assigned to the same assignee as the present invention, and O. Serra, Fundamentals of Well-Log Interpretation, Vol. 1, "The Acquisition of Logging Data", Elsevier, pps. 29-76 (1984). Both references are herein incorporated by reference.
Most prior apparatus used for determining resistivity (Rxo) of the invaded zone include an elongated support member which moves along the borehole, and a pad supported thereby which presses against the wall of the borehole. The face of the pad which contacts the wall of the borehole included an array of electrodes.
In general, two large families can be distinguished: focused apparatus and nonfocused apparatus. As used herein, the term focused apparatus refers to a system for focusing the measuring current. The family of focused apparatuses includes, in particular, the microspherical focusing apparatus described in U.S. Pat. No. 3,798,535 to Schuster and the azimuth focusing apparatus described in U.S. Pat. No. 4,117,394 to Souhaite, both assigned to the same assignee as the present invention, and both herein incorporated by reference.
The pad described in U.S. Pat. No. 4,117,394, shown in front view in FIG. 2 of the present application, is provided with an array of linear electrodes which lie substantially parallel to the longitudinal axis of the borehole and spaced apart from one another in a direction which is substantially perpendicular to the longitudinal axis. This array comprises central electrode A.sub.0 and three pairs of electrodes M.sub.1, M.sub.2 and A.sub.1 disposed symmetrically about central electrode A.sub.0. Electrode A.sub.0 delivers measuring current which penetrates into the formation and returns via the support member. The lines of the measuring current are focused by a focusing current delivered by outer pair of electrodes A.sub.1 with the value of the focusing current being continuously adjusted in such a manner as to ensure that the potential difference observed between electrodes M.sub.1 and M.sub.2 (referred to as "monitor" electrodes) is substantially zero. This servo-controlled system thus has the effect of concentrating the lines of measuring current to flow radially in a direction perpendicular to the axis of the borehole. The focusing system is said to be "active", in contrast to a "passive" focusing system based solely on guard electrodes.
Although the above-described arrangement is advantageous with respect to the size of its pad compared with the micro-spherical focusing pad of U.S. Pat. No. 3,798,535, the measurement performed by such a pad is nevertheless rapidly degraded with increasing thickness of the mud cake deposited on the wall of the borehole. For mud cake about 6 mm, the observed measurement error may be as much as 50%.